The present invention relates to a method of efficiently and selectively separating and recovering rare FP (fission products), group by group, from a nitric acid solution containing these rare FP generated from a reprocessing step of spent nuclear fuels used in nuclear power generation facilities including light water reactors and fast reactors. Further, it relates to a system for cooperation of the nuclear power generation and the fuel cell power generation by utilizing the recovered rare FP to technical fields relating with the fuel cell power generation.
In the present specification, xe2x80x9crare FPxe2x80x9d is used as a term including FP of rare metal elements such as platinum group elements (Ru (ruthenium), Rh (rhodium) and Pd (palladium)), Ag (silver), Tc (technetium), Se (selenium) and Te (tellurium).
Nitric acid solutions or radioactive process liquid wastes generated from reprocessing plants for spent nuclear fuels used in light water reactors or fast reactors contain a considerable amount of useful rare FP and, as a method of separating and recovering such rare FP, xe2x80x9ca method of separating and recovering platinum group elements, technetium, tellurium and seleniumxe2x80x9d has been proposed, for example, by Japanese Patent No. 2997266.
This prior art method comprises electrolyzing at a constant current a nitric acid solution (for example, a nitric acid solution generated from spent nuclear fuel reprocessing plants) containing one or more of elements selected from the group consisting of platinum group elements other than Pd (such as Ru, Rh and the like), Tc, Te and Se under the coexistence of Pd2+ and depositing these elements by electrolytic reduction on a cathode. In the case where a nitric acid solution containing Pd as the platinum group element is processed, there is no requirement of positively adding Pd2+. The metal elements deposited on the cathode are successively dissolved, element by element, and separately recovered, by replacing the solution to be processed in a cathode chamber with a pure nitric acid solution and controlling the electrode potential to that corresponding to the aimed element.
In the above-described prior art method of separating and recovering the rare FP, platinum group FP (Pd, Ru, Rh) deposited as solid solutions on the electrode can be separately dissolved on the basis of the difference in the dissolution potential, in principle, by controlling the dissolution potential corresponding to each of the elements. In fact, however, the method of controlling the potential involves a problem that it is difficult to maintain the reaction rate constant or that the structure for an electrolysis vessel is complicated and thus the prior art method can not be always considered as a satisfactory separation and recovery method with a engineering view point of electrolytic operation.
However, when useful rare FP contained in spent nuclear fuels can be separated and recovered selectively at a high recovery percentage, a considerable portion for the amount required to be collected and supplied from natural rare element resources can be substituted and it is possible to preserve definite natural reserves.
Furthermore, Pd, Ru and Rh as useful rare FP contained in the spent nuclear fuels have a high catalytic activity and it is expected that the demand therefor will be increased in near feature as electrode materials or as a catalyst for production and purification of fuel hydrogen for use in fuel cells.
An object of the present invention is therefore to provide a method capable of separating and recovering useful rare FP contained in spent nuclear fuels selectively and at a high recovery percentage.
Another object of the present invention is to provide a cooperation system for nuclear power generation and fuel cell power generation by utilizing the thus recovered useful rare FP as electrode materials and a catalyst for production and purification of fuel hydrogen for use in fuel cells.
The inventors of the present invention have made an earnest study with an aim of providing a method of separating and recovering useful rare FP contained in spent nuclear fuels which does not rely on an electrolysis operation at a constant potential as in the prior art method described above, and which can efficient separation and recovery of rare FP by combining operation parameters for current density and nitric acid concentration, even in the case of utilizing an electrolysis operation at a constant current which is relatively simple and convenient in view of operation and can be simplified also in view of the structure of an electrolysis vessel. Consequently, the inventors have accomplished the present invention based on the finding that separation and recovery is possible by electrolytically reducing a nitric acid solution to be processed containing useful rare FP in spent nuclear fuels using Pd2+ or Fe2+ as a catalyst, collectively depositing the rare FP on an electrode, then collectively dissolving the deposits on the electrode by electrolytic oxidation and then electrolytically reducing the solution containing dissolved deposits therein at low current density, medium current density and high current density, successively, whereby Ag.Pd group, Se.Te group and Ru.Rh.Tc group are separately deposited and recovered, group by group.
According to the present invention, there is provided a method of separating and recovering rare FP in spent nuclear fuels comprising:
a step A of supplying a nitric acid solution to be processed at a nitric acid concentration of 0.1 to 4.5 M containing one or more of rare FP selected from the group consisting of platinum group elements, Ag (silver), Tc (technetium), Se (selenium) and Te (tellurium) generated from a reprocessing step of spent nuclear fuels used in nuclear power generation facilities including light water reactors or fast reactors, to a cathode chamber together with Pd2+ (palladium) or Fe2+ (iron) as a catalyst, and conducting electrolytic reduction at a current density of 1 to 3000 mA/cm2 while supplying a pure nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to an anode chamber, thereby collectively depositing Ru (ruthenium), Rh (rhodium) and Pd as platinum group elements, and Ag, Tc, Se and Te in the nitric acid solution to be processed on the cathode;
a step B of switching the cathode to the anode and conducting electrolytic oxidation at a set potential of 1.5 to 3 V while supplying a pure nitric acid solution at a nitric acid concentration of 3 to 5 M, thereby collectively dissolving the deposits on the electrode into the pure nitric acid solution;
a step C of switching the anode to the cathode, and conducting electrolytic reduction at a current density of 1 to 25 mA/cm2 while supplying the deposit-dissolved nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the cathode chamber and supplying a pure nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the anode chamber, thereby selectively depositing Pd and Ag in the deposit-dissolved nitric acid solution on the cathode;
a step D of switching the cathode to the anode and conducting electrolytic oxidation at a set potential of 1.5 to 3 V while supplying a pure nitric acid solution at a nitric acid concentration of 3 to 5 M, thereby dissolving the deposits Pd and Ag on the electrode into the pure nitric acid solution and recovering them;
a step E of switching the anode to the cathode, and conducting electrolytic reduction at a current density of 25 to 100 mA/cm2 while supplying the deposit-dissolved and Pd.Ag-removed nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the cathode chamber and supplying a pure nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the anode chamber, thereby selectively depositing Se and Te in the deposit-dissolved and Pd.Ag-removed nitric acid solution on the cathode;
a step F of switching the cathode to the anode and conducting electrolytic oxidation at a set potential of 1.5 to 3 V while supplying a pure nitric acid solution at a nitric acid concentration of 3 to 5 M, thereby dissolving the deposits Se and Te on the electrode into the pure nitric acid solution and recovering them;
a step G of switching the anode to the cathode, and conducting electrolytic reduction at a current density of 100 to 700 mA/cm2 while supplying the deposit-dissolved and Pd.Ag.Se.Te-removed nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the cathode chamber and supplying a pure nitric acid solution at a nitric acid concentration of 0.1 to 4.5 M to the anode chamber, thereby selectively depositing Ru, Rh and Tc in the deposit-dissolved and Pd.Ag.Se.Te-removed nitric acid solution on the cathode; and
a step H of switching the cathode to the anode and conducting electrolytic oxidation at a set potential of 1.5 to 3 V while supplying a pure nitric acid solution at a nitric acid concentration of 3 to 5 M, thereby dissolving deposits Ru, Rh and Tc on the electrode into the pure nitric acid solution and recovering them.
According to the present invention, there is also provided a cooperation system for nuclear power generation and fuel cell power generation utilizing the rare FP separated and recovered by the above-described method to the following fuel cell power generation technique.
Ru and Rh are utilized as a catalyst for production of fuel hydrogen for use in fuel cells.
Ru and Rh are utilized as an electrode catalyst for use in fuel cells.
Pd is utilized as a catalyst for purification of fuel hydrogen for use in fuel cells.
Pd is utilized as a Mgxe2x80x94Pd laminated alloy for a hydrogen absorbing alloy absorbing fuel hydrogen for use in fuel cells.
A most preferred embodiment in the cooperation system for nuclear power generation and fuel cell power generation according to the present invention is to supply an electric power generated by nuclear power generation facilities including light water reactors or fast reactors as an electric power for production of fuel hydrogen for use in fuel cells, apply the separation and recovery method described above to the rare FP in spent nuclear fuels used in the nuclear power generation facilities and utilize the recovered Ru, Rh and Pd as a catalyst for production and purification of fuel hydrogen for use in fuel cells or as an electrode catalyst for use in fuel cells.